One of the prime reasons movie and music studios have ignored the Internet for open-networked multimedia content delivery, has been the lack of a technology that can support a secure digital rights management (DRM) system on a general purpose computer. The difficulty of building effective multimedia DRM stems from the fact that traditional cryptographic primitives such as encryption or scrambling do not protect audio or video signals once they are played in plain-text. This fact, commonly referred to as “the analog hole” has been responsible for the popularity of multimedia file sharing which cannot be controlled, at least technically, by the content's copyright owners.
Significantly increasing levels of multimedia piracy have put the movie and music industries under pressure to deploy a standardized anti-piracy technology for multimedia content. Initiatives, such as the Secure Digital Music Initiative (SDMI) and the Digital Versatile Disk (DVD) Copy Control Association (CCA) have been established to develop open technology specifications that protect the playing, storing, and distributing of digital music and video. Unfortunately, a DRM system that can provide a cryptographic level of multimedia protection has yet to be developed. The problem of ensuring copyright of multimedia at the client side lies in the fact that traditional data protection technologies such as encryption or scrambling cannot be applied exclusively as they are prone to the aforementioned analog hole—to digital copying or analog re-recording. The moment a potential pirate obtains an analog copy of the multimedia clip, its copyright owners, at least technically, lose control over the content's distribution. Thus, almost all modern copyright protection mechanisms tend to rely to a certain extent on tracking down pirates through watermarks: imperceptible and secret marks hidden in host signals. Two different types of protection systems have evolved over the past decade: content screening and fingerprinting.
In a typical content screening scenario, a copyright owner protects distribution rights simply by hiding a unique and secret watermark in the multimedia clips. While both the original multimedia content as well as the key used to generate the secret must be safely guarded by the owner, the marked copy can be distributed using a public communication channel such as the Internet. In general, the marked content can be distributed in analog form over the communication channel. The client's media player searches the distributed content for hidden information without needing to compare the content with the original content in order to find the hidden information. This type of watermark detection is referred to as “blind.” If the secret mark is detected, the player must verify, prior to playback, whether it has a license to play the content. Only in the case that the license is valid, does the media player play the protected clip. By default, unmarked content is considered to be unprotected and is played without any barriers. Hence, a content screening system consists of two subsystems: a watermark detector and a DRM agent which handles license management via standard cryptographic tools. An example of a DRM agent is the MICROSOFT® MEDIA PLAYER® 9 DRM system (Microsoft Corporation, Redmond, Wash.).
An important element for a content screening system is BORE-resistance. BORE is an abbreviation for “break once, run everywhere,” a typical vulnerability of content screening systems. Note that if the key used to detect the watermark is the same secret key used to mark the original clips, by breaking one player, an adversary gains access to the master secret key and as a consequence, can mark or remove marks from content at will. In order to be BORE-resistant, a content screening secret must deploy either a public-key content screening system, much like existing public-key cryptosystems (e.g., the Rivest-Shamir-Adleman system—RSA) or tamper-proof hardware and software. (See, R. L. Rivest, A. Shamir, and L. A. Adleman, “A method for obtaining digital signatures and public-key cryptosystems,” Communications of the ACM, vol. 21, no. 2, pp. 120-6, 1978.) Neither of these two goals has been fully achieved to date. But progress has been made on public-key watermarking by D. Kirovski, H. Malvar, and Y. Yacobi, in “A dual watermarking and fingerprinting system,” ACM Multimedia, pp. 372-81, 2002.
Hence, many content owners rely on fingerprinting. In a typical scenario that uses multimedia watermarking for forensic purposes, studios create a uniquely marked content copy for each individual user request. The fingerprinted copy is securely distributed to the user who plays the content using a media player which is unmodified compared to modern media players. Certain users may choose to illegally distribute this content. The media studios deploy search robots in order to find content copies on the Internet. Illegally distributed content is retrieved, and based upon the known user database as well as the original clip, media studios use forensic analysis tools to identify pirates. But even fingerprints are successfully attacked and nullified through a host of techniques that render the fingerprint unreadable or unreliable to forensic analysis tools. A great deal of time and effort can also be expended to recover a fingerprint and then correlate the fingerprint with a user from a large database of users. There is a need for multimedia fingerprints that are more imperceptible, robust, and reliable than conventional fingerprints, and for techniques that are faster and more reliable than conventional techniques for identifying pirates.